Modern dental restorative procedures have recently gravitated toward the use of polymerizable resin compositions in place of metal amalgams and other traditional dental fillers. For example, in filling cavities or other defects in the tooth's surface, many dental professionals now use polymerizable resin compositions containing inorganic glass fillers to impart desired compressive strength in place of metal amalgams. Such filled polymerizable materials are easy to apply, can be colored and shaped to correspond to the original tooth surface, and often exhibit chemical adhesion to the tooth surface when polymerized as opposed to the metallic appearance and mechanical adhesion of metal amalgams.
Although filled polymerizable composite resin compositions are in widespread use, certain problems are recognized to exist due to the nature of the compositions. For example, because such composites typically depend on polymerization reactions to effect hardening of the resin in place on the tooth surface, they exert stresses on the adjacent tooth structure due to shrinkage which occurs during polymerization as the monomers move from their free liquid state into their more dense, cross-linked polymerized state. Such shrinkage and resultant stresses are often considerable, particularly in a so-called "Class V" type restoration, wherein the restoration is being effected at the dentin-enamel junction at the cervical region of the tooth, and also in so-called "Class I" restorations such as deep cavities involving restorations contacting opposing walls of the tooth. See e.g., Feilzer et al., "Setting Stress in Composite Resins in Relation to Configuration of the Restoration", J. Dent. Res. 66(11): 1636-1639 (November, 1987) and Davidson et al. "The Competition between the Composite-Dentin Bond Strength and the Polymerization Contraction Stress, J. Dent. Res. 63(12): 1396-1399 (December 1984), the disclosures of which are hereby incorporated by reference. Such shrinkage and related stresses have been reported as causing separation of the restoration from at least the dentin surface of the tooth, leading to the creation of marginal gaps between the restorative and the adjacent tooth surface. Id. See also, Bausch et al., "Clinical significance of polymerization shrinkage of composite resins", J. PROSTHETIC DENTISTRY 48(1): 59-67 (July, 1982), the disclosure of which is hereby incorporated by reference.
In response to these problems, much effort has focused on creation of compositions and methods to reduce or eliminate shrinkage-related stresses and marginal gaps in dental restorations. Reported approaches have included reliance on the "flow" of the composite during chemical self-curing, which proceeds slowly, (See, Davidson et al., "The Relaxation of Polymerization Stresses by Flow in Dental Composites", J. Dent. Res. 63(2): 146-148 (February, 1984)) or by incremental insertion of the composite in the restorative site (See Davidson, "Resisting the Curing Contraction with Adhesive Composites", J. Prosth. Dent. 55(1): 446-447 (April 1986.) The first "flow" study did not, however, investigate composite-dentin bonding and whether "flow" would obviate gap creation, and the incremental insertion approach was disapproved as ineffective in the next reported publication by one of the same authors. Flow dissipation of shrinkage is also believed to be limited to self-cure chemical polymerization, which occurs over a period of at least several minutes, as opposed to light or heat induced polymerization, which is often completed in a matter of 1-2 minutes or less.
Others have proposed multi-step application procedures using low-viscosity, unfilled resins to seal the marginal gaps directly after initial curing of the composite, (See, Kemp-Scholte et al., "Marginal Sealing of Curing Contraction Gaps in Class V Composite Resin Restorations", J. Dent. Res. 67(5): 841-845 (May, 1988)), or use of so-called "flexible" intermediate layers of unfilled resins or light-cured glass ionomer layers applied as a thin liner layer between the tooth surface and the composite. See, Kemp-Scholte et al., "Complete Marginal Seal of Class V Resin Composite Restorations Effected by Increased Flexibility", J. Dent. Res. 69(6) 1240-1243 (June 1990). The later study reported that the better stress release liner, the glass ionomers, actually cracked and exhibited cohesive failures. Unfilled resins are also known to exhibit significant shrinkage and generally become brittle upon curing. These multi-step, multi-material approaches also introduce complexity into the dental restoration process in terms of number of steps, materials, and increase the time spent and cost incurred by the dental professional and patient in the treatment process.
However, in addition to exhibiting good adhesion and bonding, composites and other polymerizable dental restorative material must also withstand the compressive, tensile and other forces experienced by the tooth surface in the mouth. For example, considerable compressive forces are generated by contact from other teeth during chewing and other mouth movements. The restorative may also experience tensile and abrasive forces in the mouth depending on its location on or within the tooth's surfaces. In, for example, Class V restorations, shear forces are also experienced in the restoration during mastication. Such shear forces must also be absorbed and/or dissipated or the restoration may fail. See "Clinical Status of Praesens of Dentine Adhesives" pp. 113-115, the disclosure of which is hereby incorporated by reference.
There exits, therefore, a need in the art for dental composite compositions and other restorative compositions which exhibit good bond strength, good tensile and compressive strengths; and which are easy to apply and use in dental restorative procedures.